This invention relates to cast metal-matrix composite materials, and associated casting procedures and products made with the composite materials.
Metal-matrix composite materials have reinforcement in the form of fibers, whiskers, particles, or the like, embedded in a solid metallic matrix. The reinforcement imparts excellent mechanical properties to the composite material, while the metal matrix holds the reinforcement in the desired shape and form and protects it from external damage. The matrix also imparts important physical properties to the composite material such as good thermal conductivity and selected surface properties.
There are two principal techniques for preparing articles from the metal-matrix composite materials. In a casting technique, a homogeneous mixture of the solid reinforcement and the molten matrix material is prepared and then poured into a mold, where the molten matrix material solidifies. In an infiltration technique, the solid reinforcement is prepositioned in a container such as a mold, and the molten matrix material is forced or drawn into the container, where it solidifies. The casting technique is typically better suited for preparing composite materials with a relatively lower volume fraction of the reinforcement, and the infiltration technique is better suited for preparing composite materials with a relatively higher volume fraction of the reinforcement. However, in a further step the infiltrated material may be diluted by the addition of extra molten matrix metal to reach a lower volume fraction of reinforcement, so that the combination of infiltration and dilution permits the production of composite materials of low-to-medium volume fractions of reinforcement.
It is often difficult to prepare articles or products from the composite materials having high volume fractions of reinforcement by casting because, in the casting of many intricate parts, the mixture of molten metal and reinforcement particles must flow along channels in the interior of the mold. If the volume fraction of reinforcement particles is too high, the mixture of reinforcement particles and molten metal becomes too viscous to flow in the small channels or flows only very slowly. The result is that the molten metal may freeze before the channels are filled. Consequently, the mold is not fully filled and the desired article is never properly formed.
This casting problem is more acute for some types of commercially important reinforcement particles than for others. Although for some purposes different types of reinforcing particles are substantially identical, that is not the case in respect to the castability of the metal-matrix composite materials containing such particles. For example, metal-matrix composite materials having an aluminum alloy matrix and up to about 30 volume percent of silicon carbide reinforcement particles may be cast into articles with fine features. The maximum practical limit for the casting of similar products from composite materials having an aluminum alloy matrix and aluminum oxide particles is about 20-25 volume percent of aluminum oxide. These different limitations arise because of both the nature of the particles themselves and the types of matrix alloys that must be used to achieve stability and other features of the final composite material. The inability to achieve cast composite materials with higher volume fractions of aluminum oxide and other metal oxides prevents their use in important products.
Because casting of articles offers many other advantages over production of articles by infiltration, it would be desirable to find an approach whereby articles of difficult-to-cast compositions such as aluminum alloy/metal oxide reinforcement could be cast in higher volume fractions of reinforcing particulate than heretofore possible. The present invention fulfills this need, and further provides related advantages.